This invention relates to electrical metering devices, and more particularly to electronic ohmmeters and multimeters. Various type ohmmeters exist in the prior art. One of these is the "Two Wire" ohmmeter. Basically a two wire ohmmeter is comprised of an operational amplifier having an input lead and an output lead for connection to respective terminals of an unknown resistance, and a current source connected to the input of the operational amplifier. In operation, the voltage at the operational amplifier output is sensed and used as a measure of the unknown resistance. A problem, however, with this ohmmeter is that the output voltage of the operational amplifier is not an accurate indicator of the unknown resistance. This is because the two leads which connect the terminals of the unknown resistance to the input and output terminals of the amplifier have resistance which adds to the unknown resistance. And thus, the output voltage of the amplifier is proportional to the unknown resistance plus the lead resistance, and the latter term causes error.
The prior art also includes various four wire ohmmeters. One such meter includes an operational amplifier having inputs and outputs coupled via first and second leads to respective terminals of an unknown resistor. One terminal of the unknown resistor also couples to a current source through a third lead; while the other terminal of the resistor couples to a voltage measuring device through a fourth lead. A problem with this four wire meter, however, is that its current source must deliver a predetermined current regardless of the unknown resistance, and thus the current source is expensive to construct. Typically, it includes an operational amplifier, three precision resistors, and a transistor.
Another prior art four wire ohmmeter includes an operational amplifier having its input and output coupled via first and second leads to respective terminals of the unknown resistor. A current source couples to the operational amplifier input terminal; a third lead couples one terminal of the unknown resistance to ground; and a fourth lead couples the other terminal of the unknown resistance to a voltage measuring device. This device eliminates errors caused by the lead resistance, however, it requires a power supply and ground reference for the operational amplifier and current source which are separate and isolated from the power supply and ground reference for the voltage measuring device. Accordingly, this four wire ohmmeter is expensive to construct. Also, it generates error terms due to a drift between the two power supplies. Further, this type meter cannot drive capacitive loads in shunt with the fourth wire since the capacity will be parallel to the unknown resistance and will have a large time constant.
Therefore, it is one object of the invention to provide an improved ohmmeter.
Another object of the invention is to provide a four wire ohmmeter which compensates for errors due to lead resistance.
Another object of the invention is to provide a four wire ohmmeter which requires only one power supply.
Another object of the invention is to provide a four wire ohmmeter for precisely measuring resistance by auto-zeroing error voltages due to lead resistance and reference voltage offsets between the components within the meter.